Prior art force-balance accelerometers include a proof mass which is forced to the null of a position pickoff by means of a rebalancing force acting on the proof mass and opposing the force induced by the measured acceleration and, this force is proportional to the respective measured acceleration. In accelerometers fabricated using MEMS—(Micro-Electro mechanical System) technologies the balancing force is electrostatic and is generated by means of capacitive plates and/or comb drives. A typical prior art single axis accelerometer is illustrated by the cross section in FIG. 1 where a flexible hinge 1 constrains the movement of the proof mass 2 to rotation around a single axis. The proof mass movement is monitored by capacitive position-pickoff plates 3. The position pickoff has a null position which is also referred to as “electrical zero”, similarly, the hinge zero deflection position is referred to as “mechanical zero”. Ideally the electrical and mechanical zeroes coincide and no restoring force is generated by the hinge when the mass is constrained to the electrical zero. However, if there is an offset between the two zeroes then, even in the absence of acceleration, the balancing forces the mass the electrical zero will be opposed by the hinge to an extent which is proportional to the product of the offset and the hinge spring constant K. This parasitic force cannot be distinguished from inertial (acceleration) force and results in a measurement error. Such offset is practically unavoidable since the position pickoff zero and the electrical zero are differently influenced by temperature and long-term material instability. Therefore, the weaker the spring constant K the better is the temperature and time stability of the accelerometer. Stated differently, as the spring constant K. and the proof mass M constitute a mechanical resonator with natural frequency ω0 proportional to the square root of K/M this frequency should be lowered as much as possible.
The accelerometer FIG. 1 is an out-of-plane accelerometer since the proof mass is movable in a direction perpendicular the device plan, it is also referred to as hinged mass accelerometer (HMA). When implemented using MEMS technology the proof mass and hinge are integral and are made from single crystal silicon. The balancing force can be generated by the same capacitive plates 3 used as a position pickoff—see for example U.S. Pat. Nos. 5,473,946 5,006,487 and 6,105,427. To minimize its spring constant the hinge must be made as thin and as long as possible, however the practical length conflicts with the requirement of large proof mass which is required for high sensitivity. The result is a relatively short and thin hinge which is fragile and may not survive shocks. In-plane accelerometers are described in U.S. Pat. No. 6,817,942. FIG. 2 illustrates conceptually a typical in-plane accelerometers fabricated using a DRIE (Deep Reactive Ion Etching) process. Four slender springs 7 anchored at points 4 support the proof mass 6 and capacitive stationary plates 5 are interleaved with corresponding plates that extend from the proof mass to form a comb drive. Even though the springs can be made thin and long their width (the out-of-plane, Z dimension) is limited by the wafer thickness which by itself is limited by the DRIE fabrication technology. As a result, and unlike in FIG. 1 the hinge has a relatively low out-of-plane stiffness and deforms out of plane in response to Z-axis accelerations—which leads to cross-axis acceleration errors. These errors are caused both by the position pickoff and by the fact that the principal axis of the suspension becomes skewed relative to device plane.
A force-balance 3-axis accelerometer is described in reference [1] where the proof mass includes comb drives and capacitive plates for applying the forces in the X, Y and Z axes; the mass is suspended so as to minimize its rotational motions.
The concept of Levitated Mass Accelerometer (LMA) is also known. A simplified cross sectional view of a planar LMA described in references [2] and [3] with a round planar proof mass 10 is illustrated conceptually in FIG. 3. The figure also shows capacitive plates 8 that are used for generating actuation force on the proof mass and plates 9 for sensing its position in Z axis. However, in a planar LMA as in FIG. 3, the cross section of the planar proof mass is inadequate for generating significant in-plane (X-Y) electrostatic forces using reasonable voltages. The LMA in FIG. 3 is therefore passively centered in the X-Y plane using static electric field gradients rather than by force balancing loops as explained in reference [3] white only in the Z-axis is force-balanced to measure the respective acceleration. In contrast the proof mass of an Ideal 6 axis LMA should be constrained to the electrical zero position of each axis using a force-balance loops and the LMA be free from the shortcomings associated with mechanical hinges.
An error mechanism that affects any levitated proof mass but does not seem to have been recognized by prior art LMA, is due to parasitic electrostatic charge attracted to the levitated mass. Such charge is generated by surface contact between the proof mass and the enclosure prior to levitation and leads to parasitic forces that result in measurement errors. Since this charge is neither predictable nor repeatable from turn-on to turn-on, this error cannot be calibrated out. The present invention aims to provide the advantages of an ideal LMA without its parasitic charge deficiency, this is achieved by tethering the mass in a manner that substantially retains its mechanical freedom while at the same time providing an electric discharge path.